Fix KB5066835 Installation Failure on Windows 11 (Errors 0x800f0991 / 0x800f0983)

If KB5066835 fails to install on your Windows 11 PC, you’re not alone.

Many users are reporting issues such as:

• Error 0x800f0991
• Error 0x800f0983
• Update stuck or rollback (“Undoing changes”)
• Repeated installation failures

The good news is:
👉 Most of these issues can be fixed with a few reliable steps.

This guide walks you through simple, safe, and effective solutions — even if you’re not a technical user.

🚨 Common KB5066835 Installation Errors

Before fixing the issue, check what you’re experiencing.

Typical symptoms include:

• Installation fails repeatedly
• Update downloads but won’t install
• PC restarts and shows “Undoing changes”
• Error codes like:
  • 0x800f0991
  • 0x800f0983

👉 These are usually caused by:

• Corrupted update cache
• System file issues
• Component store problems
• Third-party software interference

⚡ Quick Fix (Try This First)

Before going deeper, try these quick checks:

• Restart your PC
• Make sure you have at least 10 GB free space
• Disconnect unnecessary USB devices
• Temporarily disable third-party antivirus

👉 Then try Windows Update again.

If it still fails, continue below.

🔧 Step-by-Step Fix (Follow in Order)

✅ Step 1: Run Windows Update Troubleshooter

1. Open Settings
2. Go to:
   System → Troubleshoot → Other troubleshooters
3. Run Windows Update

👉 This automatically fixes common update issues.

✅ Step 2: Repair System Files (DISM + SFC)

Open Command Prompt (Admin) or Windows Terminal (Admin) and run:

DISM /Online /Cleanup-Image /RestoreHealth
sfc /scannow

• DISM repairs the system image
• SFC fixes corrupted system files

👉 Restart your PC after completion.

✅ Step 3: Reset Windows Update Components

Stop services:

net stop wuauserv
net stop bits

Rename cache folders:

ren %systemroot%\SoftwareDistribution SoftwareDistribution.old
ren %systemroot%\System32\catroot2 catroot2.old

Restart services:

net start wuauserv
net start bits

👉 Then try updating again.

✅ Step 4: Install KB5066835 Manually (.msu Method)

If Windows Update keeps failing, install it manually:

1. Go to Microsoft Update Catalog
2. Search for KB5066835
3. Download the correct version (x64 / ARM64)
4. Run the .msu file

👉 This bypasses common Windows Update issues.

✅ Step 5: Try Advanced Fixes (If Needed)

If the problem persists:

• Disable Fast Startup
• Perform a Clean Boot
• Use System Restore
• Roll back to the previous Windows version (if stuck in loop)

👉 These steps help isolate deeper issues.

💻 IIS / Localhost Issues After KB5066835 (Developers)

Some developers reported:

• ERR_HTTP2_PROTOCOL_ERROR
• ERR_CONNECTION_RESET
• IIS / IIS Express not working
• Visual Studio debugging issues

Fixes to Try:

• Temporarily disable HTTP/2
• Reinstall IIS features
• Reset IIS configuration files
• Disable antivirus/firewall temporarily

👉 If HTTP/2 is the cause, re-enable it later after updates.

❗ Why KB5066835 Fails to Install

Here are the most common causes:

• Corrupted Windows Update cache
• Broken component store
• System file corruption
• Security software conflicts
• Network filtering or firewall issues

👉 In most cases, cache reset + DISM fixes it.

🛡️ How to Prevent This in the Future

To avoid update failures:

• Keep at least 10–20 GB free space
• Run updates regularly (don’t skip too many)
• Avoid forced shutdowns during updates
• Keep system files healthy (run SFC occasionally)
• Maintain backups

👉 Backup is your best protection.

❓ FAQ

Q1. Should I install KB5066835 now?

If your system is stable, yes.
But if you rely on IIS or development tools, consider waiting a few days.

Q2. Is it safe to install manually?

Yes. Installing via .msu is an official and safe method.

Q3. Why does Windows rollback updates?

Because something failed during installation — usually system or cache issues.

📌 Summary

If KB5066835 fails to install:

👉 Follow this order:

1. Restart + basic checks
2. Troubleshooter
3. DISM / SFC
4. Reset update cache
5. Manual install

👉 These steps fix most cases.

🔥 Final Tip

Windows updates can sometimes fail — but most issues are temporary.

👉 Stay calm, follow the steps, and avoid rushing repeated installs.

Recommended Reads

・OneNote for Windows 10 Support Ending: What’s Happening and How to Prepare (2025 Guide)

・Windows 11 25H2 for IT: WSUS Timeline, Pilot Plan, and the Enablement Package Model

・Fake CAPTCHA Scam: “I’m not a robot” Used to Spread Malware – How to Spot and Stay Safe

タグ: Update failed, KB5066835, Windows 11 October 2025 update, IIS ERR_HTTP2_PROTOCOL_ERROR, error 0x800F0991 fix